1. Field of the Invention
The present invention relates to nanoporous dielectric films and to a process for their manufacture. Such films are useful in the production of integrated circuits.
2. Description of the Related Art
As feature sizes in the production of integrated circuits approach 0.18 .mu.m and below, problems with interconnect RC delay, power consumption and crosstalk all become more significant. Integration of low dielectric constant (K) materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications partially mitigate these problems but each of the material candidates having K significantly lower than the currently employed dense silica suffer from disadvantages. A number of organic and inorganic polymers have K in the range of 2.2 to 3.5, however, these suffer from a number of problems including low thermal stability, poor mechanical properties including low glass transition temperature (T), sample outgassing, and long term reliability questions.
Another approach has been to employ nanoporous silicas which can have dielectric constants for bulk samples in the range of 1 to 3. Nanoporous silica is attractive because it employs similar precursors (e.g., TEOS, tetraethoxysilane) as used for SOG's and CVD SiO.sub.2 and because of the ability to carefully control pore size and pore size distribution. In addition to having a low dielectric constant nanoporous silica offers other advantages for microelectronics including thermal stability up to at least 500.degree. C., small pore size (&lt;&lt;microelectronics features), use of precursors (e.g., TEOS) that are widely used in the-semiconductor industry, the ability to tune dielectric constant over a wide range and deposition using similar tools as employed for conventional SOG processing. High porosity leads to a lower dielectric constant than corresponding dense materials, and additional compositions and processes may be also introduced. Materials issues include the need for having all pores significantly smaller than circuit feature sizes, the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability. Density (or the inverse, porosity) is the key nanoporous silica parameter controlling property of importance for dielectrics. Properties of nanoporous silica may be varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the pore volume decreases. The optimal porous material should have a compromise between mechanical strength and dielectric constant. Density is dependent on pore volume or porosity for a given material.
Nanoporous dielectric silica coatings can be formed by depositing a mixture of a liquid alkoxysilane precursor composition and a solvent onto a spinning silicon wafer substrate to thereby coat the substrate. The coating is typically polymerized, condensed, and cured to form a nanoporous dielectric silica coating on the substrate. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness. The preferred method for producing nanoporous dielectrics is through the use of sol-gel techniques whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. A preferred method for accelerating gel formation is to expose the alkoxysilane precursor to both water vapor and base vapor. One theory is that through continued reactions within the sol, one or more molecules within the sol may eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this point, called the gel point, the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term "gel" as used herein means an open-pored solid structure enclosing a pore fluid.
Nanoporous silica films are principally composed of silicon and oxygen in which there are pores distributed throughout the material. The pores range in size from about 0.1 nm to about 100 nm. Nanoporous silica films can be used provided that silanol groups (Si--OH) and water are excluded from the film. Silanols and water will raise the dielectric constant of the film because they are highly polarizable in an electric field. To make nanoporous films substantially free of silanols and water, an organic reagent such as hexamethyldisilazane or methyltriacetoxysilane, is optionally introduced into the pores of the film. This reagent reacts with silanols on the pore surfaces to form trimethylsilyl groups. The latter serve to mask the silanol groups and to make the film hydrophobic. The drawback to the use of trimethylsilyl groups is that the film is no longer pure SiO.sub.2. The carbon and hydrogen content may be as high as 10% by weight. In the fabrication of integrated circuits it is advantageous to use a dielectric film which will not degrade in an oxidizing environment. IC fabrication relies on highly oxidizing plasmas to remove photoresist from the top surface of insulating layers. Furthermore, oxidizing plasmas are used to deposit CVD (chemical vapor deposition) SiO.sub.2 layers onto nanoporous silica. Oxidizing plasmas will readily oxidize trimethylsilyl groups from nanoporous films and this will lead to the formation of water and silanols. In addition, oxidized silica films will easily absorb water from IC manufacturing environments. The retention of water and silanols as a result of oxidation and/or absorption of water from manufacturing environments cause two problems: a significant increase in dielectric constant and difficulty in forming low resistance metal vias (the poison via problem).
Thus, it would be desirable to produce a nanoporous silica film which has a dielectric constant .ltoreq.2.5, which contains low levels of water and which is stable to oxygen plasma as well as to other chemical solvents used in IC fabrication This can be accomplished in accordance with this invention, wherein nanoporous silica dielectric films are modified by electron beam exposure after an optional hydrophobic treatment by an organic reactant. The resulting films retain their nanoporous structures with reduced pore sizes, and initially have lower water content compared to thermally cured films, and hence have a dielectric constant lower than or the same as that of the thermally cured films. The resulting films have essentially no or a reduced amount of carbon and hydrogen after the electron beam process. These electron beam treated films are also not affected by oxygen plasma and chemical solvents, such as used in IC fabrication. The resistance to oxidizing plasma and chemical solvents results from the absence of methyl groups in the film as well as because of e-beam induced densification. Without the electron beam process, the oxygen plasma would react with the trimethylsilyl groups to form water. The water would raise the dielectric constant of the film and lead to high leakage current between metal lines. Although it has been previously suggested to form hydrophobic nanoporous films by treating the film with an organic surface modification reagent, the benefits of exposing such films to an electron beam were heretofore not known. Such prior art is exemplified by U.S. Pat. Nos. 5,494,858; 5,504,042; 5,523,615; and 5,470,802, as well as Ramos, et. al, "Nanoporous Silica for Dielectric Constant Less Than 2, ULSI Meeting, Boston, Mass., October 1996; Ramos, et. al, "Nanoporous Silica for ULSI Applications", 1997 Dielectrics for ULSI Multilevel Interconnection Conference (DUMIC), P. 106; and Jin, et. al., "Porous Xerogel Films as Ultra-Low Permittivity Dielectrics for ULSI Interconnect Applications", ULSI Meeting, Boston Mass., October 1996.